Eviction and best-effort storage

Widely available since 2021-12W3C

Every origin’s storage bucket is best-effort until the browser grants persistence, and a best-effort bucket may be deleted whole when the disk runs low, when the browser’s overall ceiling is reached, or, in Safari, when the user has not interacted with the site for seven days of browser use. The unit of deletion is the bucket, so an evicted origin loses its caches, databases, and service worker registration together.

How it works

The Storage Standard defines two bucket modes, best-effort and persistent, and requires that a bucket, when cleared, is cleared in its entirety (Storage Standard, whatwg.org). It leaves the trigger and the order to the browser, which is where the engines diverge.

Pressure-driven eviction in Chromium and Firefox

Chromium begins evicting when its overall ceiling, 80 % of total disk size, is exceeded, and Firefox when the disk fills up. Both apply a least-recently-used order: the best-effort origin visited longest ago is cleared first, then the next, until usage is back under the limit (Storage quotas and eviction criteria, developer.mozilla.org). An origin can therefore be evicted while far below its own quota: the trigger is the combined total, and the selection is recency, not size. Persistent buckets are skipped.

Safari’s seven-day rule

WebKit evicts on three triggers: exceeding the overall quota (80 % of disk for a browser app, 20 % for other apps that embed web content), system storage pressure, and Intelligent Tracking Prevention (Updates to Storage Policy, webkit.org). The ITP trigger, introduced in iOS 13.4 and Safari 13.1, deletes an origin’s script-writable storage after seven days of Safari use without user interaction on the site. The clock counts days on which Safari was used, not calendar days, and covers IndexedDB, localStorage, sessionStorage, media keys, and service worker registrations and caches (Full Third-Party Cookie Blocking and More, webkit.org). An origin is excluded from eviction while it has an open page or while its bucket is persistent.

Web apps added to the Home Screen run outside Safari and keep their own counter of days of use, so an installed app that is opened resets its clock; Apple states it does not expect first-party data in such an app to be deleted.

Private browsing

Private windows are a separate lifecycle. Chrome’s Incognito reduces the origin quota to about 5 % of disk and discards the data when the last Incognito window closes (Storage for the web, web.dev); Firefox and Safari likewise clear private-session storage when the session ends. None of this is LRU eviction, and persist() does not change it.

Observed behaviour

Eviction leaves no event in the page. The only signals are indirect, and each one is checkable without special tooling.

• navigator.storage.estimate() after eviction reports usage near zero for the origin while quota is unchanged, because the quota is a share of the disk and the bucket is simply empty.
• caches.keys() resolves to [] and indexedDB.databases() to [] (BCD api.IDBFactory.databases: Chrome 72, Firefox 126, Safari 14) for an origin whose bucket was cleared, which is how a service worker can detect that a precache it installed is gone and rebuild it on the next activate.
• Safari’s seven-day deletion also removes the service worker registration, so a visit after the deadline starts with navigator.serviceWorker.controller === null and the install event runs again.

The check below runs on page load and distinguishes “evicted” from “first visit” by a marker in localStorage, which Safari clears in the same sweep but Chromium and Firefox leave alone when they evict the quota-managed bucket.

async function detectEviction() {
  if (!('caches' in self) || !('storage' in navigator)) {
    return 'unknown'; // no quota-managed storage here: nothing to detect
  }
  const names = await caches.keys();
  const seenBefore = localStorage.getItem('shell-installed') === '1';
  if (names.length === 0 && seenBefore) {
    return 'evicted'; // bucket cleared under pressure; rebuild the precache
  }
  localStorage.setItem('shell-installed', '1');
  return names.length === 0 ? 'first-visit' : 'intact';
}

An 'evicted' result is the moment to re-run the precache and, if the app has earned it, to call navigator.storage.persist() so the next sweep skips this origin.

Field notes

Chrome DevTools, Application > Storage, reproduces the pressure case without a full disk: tick Simulate custom storage quota, enter a figure below the current usage, and the next write rejects with QuotaExceededError (What’s New in DevTools (Chrome 88), developer.chrome.com). The override lowers only this origin’s quota and does not trigger LRU eviction of other origins, so it tests the write-failure path, not the deletion path.

See also

• Storage Standard: buckets and eviction (whatwg.org)
• Updates to Storage Policy (webkit.org)
• Full Third-Party Cookie Blocking and More (webkit.org)
• StorageManager.persist() and persisted()
• StorageManager.estimate()
• Cache Storage API

Specifications

Specification	Status
Persistent Storage	W3C

• Legend
• Yes
• Partial
• Flag
• No
• Unknown

Browser / Platform	Support	Versions	Confidence	Source	Notes
Chrome (Desktop)	Yes	55	high	source	—
Chrome (Android)	Yes	55	high	source	1
Edge (Desktop)	Yes	79	high	source	2
Firefox (Desktop)	Yes	57	high	source	—
Firefox (Android)	Yes	57	high	source	3
Safari (macOS)	Yes	15.2	high	source	—
Safari (iOS)	Yes	15.2	high	source	4
Samsung Internet	Yes	6.0	high	source	5
WebView (Android)	Yes	55	high	source	6

1. Derived by browser-compat-data mirroring from Chrome.
2. Derived by browser-compat-data mirroring from Chrome.
3. Derived by browser-compat-data mirroring from Firefox.
4. Derived by browser-compat-data mirroring from Safari.
5. Derived by browser-compat-data mirroring from Chrome Android.
6. Derived by browser-compat-data mirroring from Chrome Android.

Source data: /compatibility/persistent-storage.json · Global usage: 93 % (StatCounter 2026-05)

Source: spec · MDN · Last verified 2026-10-03 · Confidence: high (computed from sources)